KNOXVILLE, Tenn. — Lawyers for an 83-year-old nun and two fellow nuclear protesters called their intrusion into a nuclear weapons plant in Oak Ridge in July a symbolic act featuring "Biblical graffiti," while prosecutors said Tuesday the act was a serious security breach that continues to disrupt operations.

The trial of Sister Megan Rice and Michael Walli, both of Washington, D.C., and Greg Boertje-Obed of Duluth, Minn., got underway in Knoxville with testimony from Steven C. Erhart, the top federal official at the Y-12 National Security Complex.

Erhart, the National Nuclear Security Administration manager at the site, said the break-in has hurt the international credibility of the weapons plant. But he also acknowledged under cross-examination that it exposed serious security flaws that have since been repaired and improved.

"It was an embarrassment from the Y-12 plant and the good people who work there," Erhart said.

But the response means "Y-12 has never been strong," he said.

Defense attorney Bill Quigley read several critical findings from a Department of Energy inspector general's report following the break-in, including that the perceived security at the plant officials liked to call the "Fort Knox of uranium" was unjustified and that a culture of complacency contributed to allowing the three elderly protesters to roam the facility for about two hours.

Erhart took issue with the complacency description.

"I believe a better term would have been the normalization of a deviation from the optimum," he said, bringing chuckles from the audience largely made up of supporters of the defendants.

The activists face prison terms of up to 20 years if convicted of sabotaging the national security complex.

The protesters call themselves "Transform Now Plowshares," a reference to the biblical phrase: "They shall beat their swords into plowshares, and their spears into pruning hooks." The loose-knit protest group has used high-profile demonstrations to draw attention to their nuclear disarmament goals and related causes around the country.

The trio splashed human blood on the walls of the fortress-like Highly Enriched Uranium Materials Facility, or HEUMF, in the Y-12 National Security Complex in Oak Ridge and painted phrases on its walls like: "Woe to the empire of blood."

Federal officials have said there was never any danger of the protesters reaching materials that could be detonated on site or used to assemble a dirty bomb, a fact stressed by defense attorneys.

Francis Lloyd, Rice's attorney, said most of the facts surrounding the case were not disputed. But he argued they amounted to symbolic acts.

"Offering bread, lighting candles, tapping with hammers on the side of the (facility) ... spraying of Biblical graffiti," Lloyd said. "What you're not going to hear is that what is kept in the HEUMF was touched or tampered with."

All three defendants have long protested U.S. military policies. Walli, a Vietnam veteran, was arrested two years earlier for trespassing at Y-12 and sentenced to eight months in federal prison.

The protesters' actions were lauded by some members of Congress, who said the incursion called attention to flawed security at Y-12, first built as part of the Manhattan Project during World War II that provided enriched uranium for the atomic bomb dropped on Hiroshima, Japan.

The first security officer to arrive told the inspector general he didn't notice the trespassers until they approached his vehicle and "surrendered." Guards who heard the protesters beating on the walls of the building with a hammer incorrectly assumed that they were construction workers, according to the report.